Verificationism was a central thesis of logical positivism, a movement in analytic philosophy that emerged in the 1920s by the efforts of a group of philosophers who sought to unify philosophy and science under a common naturalistic theory of knowledge.
Verifiability principle, a philosophical doctrine fundamental to the school of Logical Positivism holding that a statement is meaningful only if it is either empirically verifiable or else tautological (i.e.,such that its truth arises entirely from the meanings of its terms). Thus, the principle discards as meaningless the metaphysical statements of traditional philosophy as well as other kinds of statements—such as ethical, aesthetic, or religious principles—asserted as true but neither tautological nor known from experience. Such statements may have meaning in the sense of being able to influence feelings, beliefs, or conduct but not in the sense of being true or false and hence of imparting knowledge. According to the principle, then, a nontautological statement has meaning only if some set of observable conditions is relevant to determining its truth or falsity; so stated, it reflects the view that the meaning of a statement is the set of conditions under which it would be true.
Ayer was one of the logical positivists, a Viennese group of philosophers who were inspired by the theories of the early Wittgenstein and sought to answer rather than what makes a statement ‘meaningful’ as opposed to what makes it ‘true’.
Introduction of the principle of verification may be seen as an extension of Ayer’s earlier work on the classical account of knowledge. Having interpreted the “justified true belief” definition as “the right to be sure” in the article of that title in The Problem of Knowledge, 1956, Ayer has committed himself to the justification criteria, and was immediately faced with the problem of a measure of adequacy.
The principle of verification is a solution to the above problem, which, in Knowledge as having the right to be sure has been expressed in the statement “Words like ‘intuition’ and ‘telepathy’ are brought in just to disguise the fact that no explanation has been found.”
In that light, the principle of verification is an intellectually appealing, practical solution not just to the problem of finding a justification for a knowledge claim, but more importantly to the question of rationality itself. It complements the concept of knowledge, or, indeed, any statement, with a universal verification metric. From this perspective, its value lies not in the absolute correctness of the principle, but rather in the regularity of the aesthetically pleasing consequences.
The principle of verification can also be viewed as a reaction to the “hopelessness” of the search for a conclusive proof of God’s existence (or otherwise) and an objective verification criteria for rationality. If indeed the truth of every statement – and thus every knowledge claim as well – is either analytic or synthetic, with no third alternative available, then a claim of a theist, necessarily made about concepts with no horizontal, and only weak vertical relationship to the physical empirically verifiable reality, cannot possibly be assigned a true or false value.
The practical rationality of Ayer’s principle of verification has a serious drawback of narrowing the concept of knowledge beyond the minimum acceptable to most people. The favourite argument against it are the consequences of the application of the principle to itself. For, the principle of verification, being obviously non-analytic, cannot be empirically verified by means other than those used to verify any hypothesis, that is by verifying the consequences.
In other words, we cannot, by definition, find a suitable reference proposition as required by Ayer, for any such proposition must necessarily be verified and thus derived from the principle of verification. Therefore, the principle of verification needs to be taken a priori, an approach rejected earlier with reference to all non-analytic statements.